Aqueous metal quenching medium

ABSTRACT

Aqueous metal quenching media comprising vinyllactam/acrylamide copolymers

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under §119(e) to U.S. Provisional Application No. 61/414,439, filed Nov. 17, 2010.

The present invention relates to an aqueous metal quenching medium comprising a copolymer CP made up of

-   ≧0.1 and ≦70% by weight of at least one acrylamide and/or     methacrylamide of the general formula (I) [monomers A], -   ≧30 and ≦99.9% by weight of at least one N-vinyllactam of the     general formula (II) [monomers B] and -   ≧0 and ≦50% by weight of at least one further ethylenically     unsaturated compound which is different from the monomers A and     monomers B [monomers C],     where the total amounts of the monomers A, B and C add up to 100% by     weight (total amount of monomers) in copolymerized form,     where -   formula (I) is CH₂═C(R¹)—C(═O)—NR²R³ -   and -   formula (II) is

where

-   R¹ to R⁵ are each, independently of one another, hydrogen or a     linear or branched, optionally substituted C₁-C₁₀-alkyl radical and -   x is an integer from 2 to 8.

BACKGROUND OF THE INVENTION

The present invention relates to the use of a copolymer CP as component in an aqueous metal quenching medium, in particular a metal quenching bath.

The hardening of steels is a widespread method of exerting a targeted influence on the quality of a steel, for example hardness and toughness. For this purpose, the steel is firstly heated to a temperature of about 850° C. and then cooled quickly to room temperature by means of a metal quenching medium. Above a temperature of about 400° C., the steel transforms from the face-centered cubic austenite crystal structure into the desired metastable tetragonally distorted body-centered martensite crystal structure, resulting in a significant increase in the hardness of the steel. In addition to a volume increase of up to 1% by volume in the transformation from the austenite structure into the martensite structure (with transformation stresses at, in particular, the surface of a steel workpiece), excessively rapid cooling, for example when using water as quenching medium, in a temperature range below about 400° C. at the same time results in a volume contraction, as a result of which stresses arise, particularly in the interior of a steel workpiece. In the case of large components, e.g. crown gears for wind power plants, these various stresses lead to cracks and undesirable dimensional and shape changes. For this reason, water alone cannot be used as quenching medium for these steel components. An alternative is provided here by hardening oils. They allow rapid cooling down to a temperature of about 400° C. However, significantly slow cooling then occurs because of the reduced thermal conductivity of the oil. The stresses in the steel microstructure can be decreased in this way and the steel components have the desired hardness without deformation and cracks after quenching. Disadvantages of the hardening oils are the severe soot formation and smoke evolution during the quenching process and the associated risk of fire. In addition, the components have to be degreased in an additional working step after quenching. To circumvent these problems, aqueous metal quenching media which comprise water together with from about 1 to 25% by weight of a water-soluble polymer, for example polyvinyl alcohol, polyethylene glycol, polyacrylic acid sodium salt and in particular polyvinylpyrrolidone as active component have been developed.

Thus, U.S. Pat. No. 3,902,929 discloses metal quenching baths which comprise from 1 to 13% by weight of polyvinylpyrrolidone having an average molecular weight in the range from 5000 to 400 000 g/mol as water-soluble polymers.

U.S. Pat. No. Re 34,119 describes a metal quenching bath which comprises, as significant water-soluble polymer, up to 25% by volume of a polyvinylpyrrolidone having an average molecular weight in the range from 1 270 000 to 2 240 000 g/mol.

US-A 2009/65107 discloses metal quenching baths which comprise, as significant components, inorganic nanoparticles having an average diameter of from 0.2 to 10 μm, in particular sheet silicates such as talc, mica, montmorillonite, hectorite or saponite, and a water-soluble polymer such as polyalkylene glycol, polyvinylpyrrolidone, polyacrylamide or polyvinyl alcohol.

US-A 2009/95384 relates to metal quenching baths based on vinylpyrrolidone/vinylcaprolactam copolymers mixed with substituted oxazoline polymers, polyalkylene glycols and/or polyvinylpyrrolidones.

It was therefore an object of the present invention to provide copolymers as component for aqueous metal quenching media, which in the case of a metal object having a temperature of about 850° C., lead to significantly slower cooling compared to polyvinylpyrrolidone at the same concentration.

BRIEF SUMMARY OF THE INVENTION

The object was achieved by the use of a copolymer CP as component in an aqueous metal quenching medium and by an aqueous metal quenching medium comprising a copolymer CP as component.

The copolymers CP used according to the invention are made up of

-   ≧0.1 and ≦70% by weight of at least one acrylamide and/or     methacrylamide of the general formula (I) [monomers A], -   ≧30 and ≦99.9% by weight of at least one N-vinyllactam of the     general formula (II) [monomers B] and -   ≧0 and ≦50% by weight of at least one further ethylenically     unsaturated compound which is different from the monomers A and     monomers B [monomers C],     where the total amounts of the monomers A, B and C add up to 100% by     weight (total amount of monomers) in copolymerized form,     where -   formula (I) is CH₂═C(R¹)—C(═O)—NR²R³ -   and -   formula (II) is

where

-   R¹ to R⁵ are each, independently of one another, hydrogen or a     linear or branched, optionally substituted C₁-C₁₀-alkyl radical, for     example methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,     tert-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl     and isomeric compounds thereof, optionally substituted by one or     more functional group(s) such as hydroxy, amino, alkoxy such as     methoxy or ethoxy, alkoxycarbonyl such as methoxycarbonyl or     ethoxycarbonyl, halogen such as fluorine, chlorine or bromine     groups, and -   x is an integer from 2 to 8.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts cooling curves for several quenching media and hardening oil.

DETAILED DESCRIPTION OF THE INVENTION

The radicals R¹ to R⁵ are each advantageously hydrogen or an unsubstituted linear or branched C₁-C₁₀-alkyl radical.

In the monomers A, R¹ is advantageously hydrogen and/or methyl, with R¹ particularly preferably being hydrogen. R² and R³ are advantageously not both hydrogen. R² and/or R³ are each advantageously an unsubstituted C₁-C₄-, in particular C₂-C₄-alkyl radical such as ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl, with isopropyl and tert-butyl being particularly preferred. It is particularly advantageous for only one of the radicals R² and R³ to be a C₂-C₄-alkyl radical. Particular preference is given to using N-isopropylacrylamide and/or N-tert-butyl-acrylamide as monomers A. Of course, a copolymer CP can also comprise mixtures of various monomers A in copolymerized form.

The total amount of monomers A in the copolymers CP used according to the invention is ≧0.1 and ≦70% by weight, preferably ≧5 and ≦40% by weight and particularly preferably ≧10 and ≦35% by weight, in copolymerized form.

N-Vinyllactams of the general formula (II) are used as monomers B. Here, R⁴ and R⁵ can each be, independently of one another, hydrogen and/or C₁-C₁₀-alkyl. Hydrogen and methyl are preferred as R⁴ and R⁵. Particular preference is given to hydrogen. The monomer B frequently comprises no methyl group or a total of only one methyl group.

According to the invention, x is an integer from 2 to 8, frequently 3, 4, 5, 6 or 7. In particular, x is an integer from 3 to 5, but particularly advantageously 3 or 5.

Examples of monomers B are the N-vinyl derivatives of the following lactams: 2-pyrrolidone, 2-piperidone, ε-caprolactam and alkyl derivatives thereof, for example 3-methyl-2-pyrrolidone, 4-methyl-2-pyrrolidone, 5-methyl-2-pyrrolidone, 3-ethyl-2-pyrrolidone, 3-propyl-2-pyrrolidone, 3-butyl-2-pyrrolidone, 3,3-dimethyl-2-pyrrolidone, 3,5-dimethyl-2-pyrrolidone, 5,5-dimethyl-2-pyrrolidone, 3,3,5-trimethyl-2-pyrrolidone, 5-methyl-5-ethyl-2-pyrrolidone, 3,4,5-trimethyl-2-pyrrolidone, 3-methyl-2-piperidone, 4-methyl-2-piperidone, 5-methyl-2-piperidone, 6-methyl-2-piperidone, 6-ethyl-2-piperidone, 3,5-dimethyl-2-piperidone, 4,4-dimethyl-2-piperidone, 3-methyl-ε-caprolactam, 4-methyl-ε-caprolactam, 5-methyl-ε-caprolactam, 6-methyl-ε-capro-lactam, 7-methyl-ε-caprolactam, 3-ethyl-ε-caprolactam, 3-propyl-ε-caprolactam, 3-butyl-ε-caprolactam, 3,3-dimethyl-ε-caprolactam or 7,7-dimethyl-ε-caprolactam. Of course, a plurality of different monomers B can also be present in copolymerized form in the copolymer CP.

N-Vinylpyrrolidone and/or N-vinylcaprolactam are particularly advantageously used, with N-vinylpyrrolidone being particularly preferred as monomer B.

The total amount of monomers B in the copolymers CP used according to the invention is ≧30 and ≦99.9% by weight, preferably ≧60 and ≦95% by weight and particularly preferably ≧65 and ≦90% by weight, in copolymerized form.

Possible monomers C are all ethylenically unsaturated compounds which differ from the monomers A and B and can be free-radically copolymerized with these in a simple manner, for example vinylaromatic monomers such as styrene, α-methylstyrene, o-chlorostyrene or vinyltoluenes, vinyl halides such as vinyl chloride or vinylidene chloride, esters of vinyl alcohol and monocarboxylic acids having from 1 to 18 carbon atoms, e.g. vinyl acetate, vinyl propionate, vinyl n-butyrate, vinyl neodecanoate, vinyl laurate and vinyl stearate, esters of α,β-monoethylenically unsaturated monocarboxylic and dicarboxylic acids preferably having from 3 to 6 carbon atoms, in particular acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid, with alkanols generally having from 1 to 12, preferably from 1 to 8 and in particular from 1 to 4, carbon atoms, especially methyl, ethyl, n-butyl, isobutyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl and 2-ethylhexyl acrylates and methacrylates, dimethyl or di-n-butyl fumarate and maleate, nitriles of α,β-monoethylenically unsaturated carboxylic acids, e.g. acrylonitrile, methacrylonitrile, fumaric dinitrile, maleic dinitrile, ethers derived from vinyl alcohol and alcohols having from 1 to 18 carbon atoms, for example n-butyl, cyclohexyl, dodecyl, ethyl, 4-hydroxybutyl or octadecyl vinyl ethers, and also C₄₋₈-conjugated dienes such as 1,3-butadiene (butadiene) and isoprene. These monomers generally have only a moderate to low solubility in water under normal conditions [20° C., 1 atm (absolute)]. Normally, the above-mentioned monomers C are only used as modifying monomers C in amounts of ≦10% by weight, preferably ≦5% by weight and particularly preferably ≦3% by weight, in each case based on the total amount of monomers C. However, it is advantageous not to use any monomers C of this type.

Monomers C which have an increased solubility in water under the abovementioned conditions are those which comprise either at least one carboxylic acid or sulfonic acid group or the corresponding anion thereof and/or at least one amino, ureido or N-heterocyclic group and/or the ammonium derivatives thereof which are protonated or alkylated on the nitrogen. Examples which may be mentioned are acrylic acid, methacrylic acid, fumaric acid, maleic acid and itaconic acid, vinylsulfonic acid, styrenesulfonic acid and water-soluble salts thereof, advantageously alkali metal or ammonium salts thereof, and also 2-vinylpyridine, 4-vinyl-pyridine, 2-vinylimidazole, 2-(N,N-dimethylamino)ethyl acrylate, 2-(N,N-dimethylamino)ethyl methacrylate, 2-(N,N-diethylamino)ethyl acrylate, 2-(N,N-diethylamino)ethyl methacrylate, 2-(N-tert-butylamino)ethyl methacrylate and 2-(1-imidazolin-2-onyl)ethyl methacrylate. Normally, the abovementioned monomers C are used as main monomers C in amounts of ≧50% by weight, preferably ≧80% by weight and particularly preferably ≧90% by weight, in each case based on the total amount of monomers C. However, it is advantageous not to use any monomers C of this type.

Monomers C which usually increase the internal strength of the films of a polymer matrix normally have at least one hydroxy group, at least one epoxy group or at least one carbonyl group or at least two nonconjugated ethylenically unsaturated double bonds. Examples are monomers having two vinyl radicals, monomers having two vinylidene radicals and monomers having two alkenyl radicals. The diesters of dihydric alcohols with α,β-monoethylenically unsaturated monocarboxylic acids are particularly advantageous, and among these acrylic acid and methacrylic acid are preferred. Examples of such monomers having two nonconjugated ethylenically unsaturated double bonds are alkylene glycol diacrylates and dimethacrylates, e.g. ethylene glycol diacrylate, 1,2-propylene glycol diacrylate, 1,3-propylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycol diacrylate and ethylene glycol dimethacrylate, 1,2-propylene glycol dimethacrylate, 1,3-propylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylate and also divinylbenzene, N,N′-divinyl-ethyleneurea, vinyl methacrylate, vinyl acrylate, allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, methylenebisacrylamide, cyclopentadienyl acrylate, triallylcyanurate or triallyl isocyanurate. It can also be advantageous to use hydroxy-functionalized alkyl acrylates or methacrylates, for example 2-hydroxyethyl, 2-hydroxypropyl or 4-hydroxybutyl acrylate or methacrylate. The abovementioned crosslinking monomers C are frequently used in amounts of ≦10% by weight, but preferably in amounts of ≦3% by weight, in each case based on the total amount of monomers C. However, particular preference is given to using no monomers C of this type.

The total amount of monomers C in the copolymers CP used according to the invention is ≧0 and ≦50% by weight, preferably ≧0 and ≦20% by weight and particularly preferably ≧0 and ≦10% by weight, in copolymerized form. In a preferred embodiment, the copolymers CP do not comprise any monomers C in copolymerized form.

Therefore, in the copolymers CP used according to the invention, R¹, R⁴ and R⁵ are each preferably hydrogen, x is an integer from 3 to 5 and R² and R³ are not both hydrogen. R² and/or R³ are each particularly advantageously an unsubstituted C₂-C₄-alkyl radical, in particular a tert-butyl radical or isopropyl radical. The aqueous quenching media of the invention therefore advantageously comprise copolymers CP which comprise N-tert-butylacrylamide and/or isopropylacrylamide as monomers A in copolymerized form.

The preparation of the copolymers CP used according to the invention is not critical and is carried out by polymerization, preferably by free-radical polymerization, of the monomers A and B and optionally C. Here, the free-radical polymerization of the monomers A and B and optionally C can be carried out in the form of a bulk polymerization or in the form of a solution polymerization (see, for example S. Dincer, Z. Rzaev, E. Piskin, J. Poly. Research 2006, 13, pages 121 to 131; F. Eeckman, A. Moes, K. Amighi, Eur. Polym. J. 2004, 40, pages 873 to 881, and I. Nam, J. Bae, K. Jee, J. Lee, K. Park, Macromol. Res. 2002, 10, pages 115 to 121).

In the preparation of the copolymers CP according to the invention, it is possible to place, in each case optionally, a partial amount or the total amount of monomers A and B and optionally C in the polymerization vessel at the beginning. However, it is also possible to introduce, in each case, the total amount or any remaining amount of monomers A and B and optionally C during the polymerization reaction. The respective total amount or any remaining amount of monomers A and B and optionally C can be introduced discontinuously in one or more portions or continuously at constant or changing flows into the polymerization vessel. Of course, the invention also encompasses copolymers CP which are obtained by the stepwise or gradient mode of operation.

The copolymers CP used according to the invention advantageously have a K value in the range ≧30 and ≦150 (determined at 25° C. in a 1% strength aqueous solution). The copolymers particularly advantageously have a K value in the range ≧60 and ≦120. The setting of the K values in the preparation of the copolymers CP is well known to those skilled in the art and is advantageously achieved by free-radically initiated solution polymerization in the presence of free-radical chain transfer compounds, also known as free-radical chain regulators. A person skilled in the art will be familiar with the determination of the K value (see, for example, H. Fikentscher, “Systematik der Cellulosen aufgrund ihrer Viskosität in Lösung” in Cellulose-Chemie 13 (1932), pages 58 to 64 and pages 71 to 74, or Encyclopedia of Chemical Technology, Vol. 21, 2nd edition (1970), pages 427 and 428).

The copolymers CP used according to the invention generally have weight-average molecular weights of ≧30 000 and ≦3 000 000 g/mol and advantageously ≧50 000 and ≦2 500 000 g/mol. The weight-average molecular weights are determined by gel permeation chromatography using defined polyvinylpyrrolidone standards (for general methods, see, for example, M. J. R. Cantow et al., Journal of Polymer Science: Part A-1, Vol. 5, 1967, pages 1391 to 1394).

The copolymers CP used according to the invention also have a solubility at 23° C. and 1 atm (1.013 bar absolute) of ≧10 g, advantageously ≧50 g, per 100 g of deionized water. In a preferred embodiment, copolymers CP which under the abovementioned conditions give a solution with deionized water in any ratio are used.

The copolymers CP used according to the invention are advantageously used in the form of aqueous solutions and/or powder.

The aqueous metal quenching media of the invention comprise, as significant component, at least one of the abovementioned copolymers CP. The aqueous metal quenching media of the invention particularly advantageously comprise copolymers CP which in copolymerized form are made up of

-   ≧5 and ≦40% by weight of monomers A and -   ≧60 and ≦95% by weight of monomers B.

The aqueous metal quenching media of the invention comprise at least one of the abovementioned copolymers CP in a total amount of 0.1 and 10% by weight, advantageously ≧0.1 and ≦5% by weight and particularly advantageously ≧0.5 and ≦3% by weight.

Apart from water and at least one copolymer CP, the aqueous metal quenching media can further comprise customary additives with which a person skilled in the art will be familiar, in particular active compounds to combat microorganisms, for example biocides, bactericides or fungicides, e.g. polyaminopropylbiguanide (obtainable from Arch under the trade name Cosmocil® CQ), paraformaldehyde, glutaraldehyde, phenoxyethanol, 2,4-dichlorobenzyl alcohol, 2,3-dibromo-3-nitrilopropionamide, 5-chloro-2-methyl-2H-isothiozol-3-one, antifoams such as silicone oils, fatty alcohol alkoxylates, alcohol alkoxylates, carboxylic esters or phosphoric esters and/or corrosion inhibitors such as borax, sodium nitrite, amines, ammonium salts of organic acids, phosphoric esters, alcohol alkoxylates or 2-butyne-1,4-diol, and also further compounds such as ethanolamine, polyalkylene oxides, polyethylene glycols. In general, the total content of the customary additives in the aqueous metal quenching medium is in the range ≧0.1 and ≦20% by weight.

The aqueous metal quenching media of the invention therefore advantageously comprise

-   ≧70 and ≦99.8% by weight of water -   ≧0.1 and ≦10% by weight of copolymer CP and -   ≧0.1 and ≦20% by weight of customary additives.

A person skilled in the art will also be familiar with the way in which the aqueous metal quenching media of the invention are employed. Thus, for example, a metal object to be cooled (quenched) can be sprayed or sprinkled with an aqueous metal quenching medium according to the invention. It is also possible to pass a metal object to be quenched through a down-flowing liquid film of aqueous metal quenching medium. However, a metal object to be quenched is particularly advantageously dipped (quenched) in an aqueous metal quenching medium according to the invention, as a result of which heat energy is removed particularly efficiently, which is why the use of copolymer CP in such metal quenching baths is particularly preferred.

The use according to the invention of copolymers CP makes available aqueous metal quenching media, in particular aqueous metal quenching baths, by means of which, in comparison with corresponding metal quenching media/metal quenching baths based on polyvinylpyrrolidone, metallic objects, in particular objects made of iron-, aluminum- and/or titanium-comprising alloys, but in particular steels, can be cooled significantly more slowly, with the cooling curves being in the region between the cooling curves obtained in polyvinylpyrrolidone and in hardening oil based on mineral oil. It is also of importance that the costly addition of inorganic nanoparticles or other copolymers can be dispensed with. Furthermore, any residual amounts of copolymers CP adhering to the metal objects after cooling can be rinsed off with water in a simple manner.

The invention is illustrated by the following nonlimiting examples.

EXAMPLES 1 Preparation of the Copolymers CP 1.1 Copolymer CP1

30.0 g of N-isopropylacrylamide, 120 g of N-vinylpyrrolidone and 570 g of deionized water were placed at 20-25° C. (room temperature) and under a nitrogen atmosphere (1.013 bar absolute) in a 2 l four-necked flask equipped with anchor stirrer, reflux condenser, introduction facility and nitrogen inlet. While stirring, the initial charge was heated to 75° C., a suspension comprising 0.3 g of Wako® V59 (free-radical initiator from Wako Pure Chemical Industry, Ltd.) and 12.0 g of deionized water was then added all at once and the polymerization mixture was stirred at the abovementioned temperature for another 3 hours. The polymerization mixture was subsequently heated to 85° C. and stirred at this temperature for a further 90 minutes. The polymerization mixture was then brought to a pH of <4 (pH measurement by means of a sensor at room temperature) by means of formic acid and stirred at 85° C. for another 3 hours. The polymerization mixture was subsequently cooled to room temperature and brought to a pH of 7 by addition of triethanolamine. This gave a polymer solution having a solids content of 17% by weight. The K value of the copolymer CP1 was 95.

The solids content was generally determined by drying a defined amount of the aqueous polymer solution (about 1 g) to constant weight in an aluminum crucible having an internal diameter of about 5 cm at 120° C. in a drying oven (about 2 hours). Two separate measurements were carried out in each case. The values reported in the examples represent the mean of the respective two measured results.

The determination of the Fikentscher K value was generally carried out at 25° C. using an instrument from Schott, Mainz (capillary: Micro-Ostwald; type: MO-Ic). The aqueous copolymer solutions obtained were diluted with deionized water in such a way that the resulting homogeneous solutions had a polymer content of 1.0% by weight.

1.2 Copolymer CP2

The preparation of the copolymer CP2 was carried out in a manner analogous to the preparation of the copolymer CP1 except that 45.0 g of N-isopropylacrylamide, 105 g of N-vinyl-pyrrolidone and 570 g of deionized water were used.

This gave an aqueous copolymer solution having a solids content of 17% by weight. The K value of the copolymer CP2 was 100.

1.3 Copolymer CP3

1.5 g of N-tert-butylacrylamide, 13.5 g of N-vinylpyrrolidone and 570 g of deionized water were placed at room temperature and under a nitrogen atmosphere (1.013 bar absolute) in a 2 l four-necked flask equipped with anchor stirrer, reflux condenser, introduction facility and nitrogen inlet. While stirring, the initial charge was heated to 75° C. and a suspension comprising 0.3 g of Wako® V59 and 12.0 g of deionized water was then added all at once. Commencing at the same time, a mixture of 121.5 g of N-vinylpyrrolidone and 13.5 g of N-tert-butylacrylamide was metered into the initial charge over a period of 3 hours at a constant flow rate while stirring. The polymerization mixture was subsequently heated to 85° C. and stirred at this temperature for a further 90 minutes. A solution comprising 0.5 g of Wako® V59 and 2.0 g of isopropanol was subsequently added all at once to the polymerization mixture and the mixture was after-polymerized at 85° C. for 2 hours while stirring. The polymerization mixture was subsequently brought to a pH of <4 by means of formic acid (pH measurement of a sample at room temperature) and stirred at 85° C. for another 3 hours. The polymerization mixture was subsequently cooled to room temperature and brought to a pH of 7 by addition of triethanolamine. This gave a copolymer solution having a solids content of 20% by weight. The K value of the copolymer CP3 was 75.

1.4 Copolymer CP4

The preparation of copolymer CP4 was carried out in a manner analogous to the preparation of the copolymer CP3 except that 3.0 g of N-tert-butylacrylamide, 12.0 g of N-vinylpyrrolidone and 570 g of deionized water were initially charged and a mixture of 108 g of N-vinylpyrrolidone and 27.0 g of N-tert-butylacrylamide was metered in.

This gave an aqueous copolymer solution having a solids content of 20% by weight. The K value of the copolymer CP4 was 60.

1.5 Comparative polymer CCP

A 20% strength by weight aqueous solution of polyvinylpyrrolidone having a K value of 97 (Luvitec® K 90; sales product of BASF SE) was used as comparative polymer CCP.

1.6 Hardening oil

Likewise for comparison, the reference oil “Bellini FN” was used as hardening oil based on mineral oil.

2 Determination of the Cooling Behavior 2.1 Production of the Quenching Media QCP1 to QCP4 and QCCP/Hardening Oil

The quenching media QCP1 to QCP4 and QCCP were produced by diluting the aqueous solutions of the corresponding copolymers CP1 to CP4 and CCP with deionized water to a solids content of 1% by weight. The hardening oil based on mineral oil was used directly without further treatment.

2.2 Determination of the Cooling Rates

The cooling rates were determined using the Smart Quench testing system from Swerea IVF, Sweden. The cylindrical standard test body of distortion-resistant Inconel® 600 having a diameter of 12.5 mm and a length of 60 mm and a thermocouple in the center of the body was used and was heated to 860° C. and subsequently dipped into the associated circulation apparatus comprising 1.5 liters of the appropriate quenching medium QCP1 to QCP4 and QCCP or hardening oil at a temperature of 23° C. The stirring rate in the circulation apparatus was set to 1000 rpm during the measurements. The cooling measurements were started automatically at 850° C. The total measurement time was in each case 20 seconds.

The corresponding cooling curves are summarized in FIG. 1.

It can clearly be seen from the cooling curves obtained that use according to the invention of the copolymers QCP1 to QCP4 in aqueous quenching media results in a cooling behavior which is in a region between the cooling behavior given by an aqueous quenching medium based on a polyvinylpyrrolidone homopolymer and a hardening oil based on mineral oil. 

1. An aqueous metal quenching medium, comprising a copolymer comprising in polymerized form: (a) ≧0.1 and ≦70% by weight of a monomer (a) consisting of at least one acrylamide, methacrylamide, or both, of formula (I) CH₂═C(R¹)—C(═O)—NR²R³  (I); (b) ≧30 and ≦99.9% by weight of a monomer (b) consisting of at least one N-vinyllactam of formula (II)

and (c) ≧0 and ≦50% by weight of a monomer (c) consisting of at least one further ethylenically unsaturated compound which is different from the monomer (a) and the monomer (b) wherein the total amount of the monomer (a) +the monomer (b) +the monomer (c) equals 100% by weight in copolymerized form, R¹ to R⁵ are each, independently of one another, hydrogen or a linear or branched, optionally substituted, C₁-C₁₀-alkyl radical, and x is an integer from 2 to
 8. 2. The medium of claim 1, comprising a copolymer comprising in polymerized form: (a) ≧5 and ≦40% by weight of the monomer (a); and (b) ≧60 and ≦95% by weight of the monomer (b).
 3. The medium of claim 1, wherein R¹, R⁴ and R⁵ are each hydrogen, x is an integer from 3 to 5, and R² and R³ are not both hydrogen.
 4. The medium of claim 1, wherein at least one of R² and R³ is an unsubstituted C₂-C₄-alkyl radical.
 5. The medium of claim 1, wherein at least one of R² and R³ is a tert-butyl radical or an isopropyl radical.
 6. The medium of claim 1, comprising a copolymer comprising in polymerized form the monomer (a) consisting of N-tert-butylacrylamide, N-isopropylacrylamide, or both.
 7. The medium of claim 1, wherein the copolymer has a Fikentscher K value of ≧30 and ≦150.
 8. The medium of claim 1, wherein a total amount of the copolymer is ≧0.1 and ≦10% by weight.
 9. A process for cooling a metal object, the process comprising contacting a metal object with the aqueous metal quenching medium of claim
 1. 10. An aqueous metal quenching bath, comprising the aqueous metal quenching medium of claim
 1. 11. The medium of claim 2, wherein R¹, R⁴ and R⁵ are each hydrogen, x is an integer from 3 to 5, and R² and R³ are not both hydrogen.
 12. The medium of claim 2, wherein at least one of R² and R³ is an unsubstituted C₂-C₄-alkyl radical.
 13. The medium of claim 3, wherein at least one of R² and R³ is an unsubstituted C₂-C₄-alkyl radical.
 14. The medium of claim 2, wherein at least one of R² and R³ is a tert-butyl radical or an isopropyl radical.
 15. The medium of claim 2, comprising a copolymer comprising in polymerized form the monomer (a) consisting of N-tert-butylacrylamide, N-isopropylacrylamide, or both.
 16. The medium of claim 2, wherein the copolymer has a Fikentscher K value of ≧30 and ≦150.
 17. The of claim 2, wherein a total amount of the copolymer is ≧0.1 and ≦10% by weight.
 18. A process for cooling a metal object, the process comprising contacting a metal object with the aqueous metal quenching medium of claim
 2. 19. An aqueous metal quenching bath, comprising the aqueous metal quenching medium of claim
 2. 